The game's story revolves around the appearance of two alien races in Terran space and each race's attempts to survive and adapt over the others. The player assumes three roles through the course of the three campaigns: a Confederate colonial governor who becomes a revolutionary commander, a Zerg cerebrate pushing forward the species' doctrine of assimilation, and a Protoss fleet executor tasked with defending the Protoss from the Zerg.
The expansion's story continues only days after the conclusion of the original game. It starts with the Protoss' struggle to ensure the survival of their species and continues with the intervention of the United Earth Directorate into local Terran affairs. The livelihood of both the Protoss and the previously silent Earth government is then threatened by the ever-increasing power of Sarah Kerrigan and her Zerg broods. In addition, the expansion introduces new features and improvements. A total of seven new units with different functions and abilities are included, the artificial intelligence behavior was modified, new graphical tilesets for terrain were added, and the game's level editor received improved scripting tools to facilitate cut scenes with the in-game engine.